In many applications it is critically important to minimize the cost of documents that are capable of carrying changeable magnetically encoded information. For example, several transportation systems use small cardboard tickets bearing a magnetic stripe that serves to account for the current value of the ticket. A system may be based on single-use tickets or on multiple-use tickets. In the latter case, the purchase value of the ticket is decremented with use, the instant value being printed on the ticket and recorded on the magnetic stripe. Of course, other information also may be recorded on the magnetic stripe.
In contrast to conventional plastic credit cards (bearing a magnetic stripe), transportation cards or decrementing tickets normally have a very short life involving a limited number of uses. As a consequence and in view of the great volume of paper tickets consumed, any significant reduction in the cost of producing the tickets results in a considerable economy.
It has been proposed to produce inexpensive magnetic stripe documents by printing the stripe on paper. While the technique has been used, accomplishing a reliable recording medium generally imposes severe constraints on the production process. Accordingly, a considerable need exists for an improved method and apparatus involving a document bearing a magnetic medium of marginal quality. For example, as contemplated by the present invention, a magnetic stripe may be applied on paper by a high speed printing process to provide a marginal magnetic recording medium which may be reliably used to carry changeable digital information in accordance with the present invention.
In general, the system of the present invention involves defining a pair of record tracks on a marginal magnetic stripe, each track defining a series of binary bit locations. In accordance with the disclosed embodiment, "one" digits preliminarily are recorded in each bit location of each track. Associated pairs of the bit locations (one from each track) are utilized to record information if the pair possesses a standard of record quality. Pairs of bit locations that do not meet the standard of record quality are cleared of data and ignored by the system. Note that active record areas need not be contiguous but may occur as distinct sections of the magnetic stripe. Accordingly, some reduction in the relative overall speed of reading and recording is to be expected.
Recognizing the existence of several magnetic recording formats for digital information, the disclosed embodiment utilizes an exemplary pulse recording technique along with selective erasure. That is, pulses representing "one" digits are recorded in each bit location of both tracks. The recorded tracks are then sensed to measure the recording quality of the bit locations on the basis of pulse amplitude. If either bit location in an associated pair is defective, the pair is erased (cleared to "zero" or greater) and disregarded.
If a pair of bit locations meets a standard of record quality, three possible situations are used to record information. Specifically, as possible situations, the recorded pulse may be erased in either of the two bit locations, or both pulses may be permitted to remain. Thus, based on the three possible data situations, a trinary code may be employed as disclosed in detail below. Other formats are practical as disclosed below.